Intake fraction
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<section begin=glossary />
- Intake fraction (also iF) is the fraction of emission that is eventually inhaled or ingested by someone in the target population (population of interest) - integrated over time and space.
- Intake fraction can be estimated with different methods, such as
- iF based on measured concentration fields
- iF based on exposure monitoring
- iF based on shortcuts
<section end=glossary />
- The text on this page is taken from an equivalent page of the IEHIAS-project.
Contents
Scope
Purpose
The purpose of intake fraction is to provide a representation of the emissions-to-intake relationship. This is a significant part of the risk assessment of chemicals. Quantification of this relationship provides an indication of the potential impact of emissions on exposed populations and allows for the determination of the effect of source control on health outcomes. Thus, an important tool for risk assessment is the derivation of a value that relates emissions to exposure in an efficient manner for both screening level assessments and policy comparisons. A useful attribute of iF is that it can be applied under conditions of very limited data, so long as the underlying principles are known. It is thus especially useful at the screening stage in impact assessments.
Boundaries
Intake fraction is subject to the same uncertainties associated with any modelling assessment (e.g. parameter uncertainty, model specification uncertainty). There are, therefore, several assumptions and limitations to be aware of when using intake fraction.
- There is an assumption that the relationship between emissions and concentration (and intake) is linear. Intake fraction has been less frequently applied for reactive or secondary pollutants.
- With an aggregate measure such as iF, one must be careful to include changes over time in the model.
- Difficulties arise in how to deal with multiple exposures - i.e. repeated intake of the same pollutant entity. On the one hand, one needs to be careful not to double-count exposure:
- long-lived substances, especially, may recycle though the environment and be available for multiple intake;
- some substances (e.g. dioxins) may be passed between mother and child.
On the other hand, such recycling is still a part of the impact of the emissions that perhaps should not be ignored.
Method description
Input
Intake fraction requires two types of inputs, both of which can be derived from measurements, modelling, or a combination of the two. Since iF is a fraction, one input is the numerator, which is the population or individual intake value, generally in units of mass. The other input is the denominator, which is the emissions (in mass) from the source(s). The units may also be in rates (e.g. mass/time). The intake and emissions must be of the same units.
Output
The output comprises a dimensionless number that summarises, for every unit emission of a pollutant from a source or source type, the fraction that is taken in by the exposed population: e.g. for every tonne of benzene emitted from motor vehicles in a given city, 1 gram is inhaled by the exposed population in that city.
Method
Intake fraction (iF) estimates how much of a unit emission of a pollutant is taken up by an exposed population. In other words, iF is the integrated incremental intake of a pollutant released from a source or source category (such as mobile sources, power plants, or refineries) summed over all exposed individuals during a given exposure time, per unit of emitted pollutant (Bennett et al. 2002a).
(1) Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): iF=\frac{ \sum_{people, time} \text{mass intake of pollutant by an individual}}{\text{mass release into the environment}}
Practically speaking, this is usually quantified as
(2) Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): iF=\frac{\text{Concentration}_{\text{Source}} \times \text{Population} \times \text{Intake rate}}{\text{Emissions}}
An intake fraction of 10-6 can be interpreted as for every kg of pollutant released into the environment 1 μg of the pollution will be taken up by the exposed population (Bennett et al. 2002a).
Three intake routes are included in the concept: intake through ingestion or inhalation, and dermal uptake. The different routes are related to the total intake fraction according to the following relationship for all exposure pathways (Bennett et al. 2000b).
iF(total) = iF(inhalation) + iF(ingestion) + iF(dermal) [3]
Bennett and co-workers suggest the following notation: iF(route, media, subpopulation), where route refers to ingestion, inhalation, dermal uptake or total, media refers to release to air, water and soil, and subpopulation refers to exposed group - e. g. workers, residents or all exposed. While population iF is useful in determining large-scale impacts of a pollutant, the evaluation of the distribution of individual intake fractions throughout a population space can also provide useful information. The total intake fraction can then be calculated as the sum of all of the individual intake fractions (iFi) for an exposed population (Bennett et al. 2002a).
As exposures to pollution are rarely evenly distributed, the effectiveness of control policies should account for the factors that lead to particularly high exposures. The distribution of individual intake fractions across time and space and various activities and microenvironments can provide such information. Generally iF should be applied for situations with a fairly long time frame. Calculations can be made for short periods also, but these are less useful, at least for screening level purposes. Most commonly, iF is calculated as an annual average or for a lifetime. The numerator of iF requires an estimation of population or individual intake, derived from multiplying the media concentration or exposure of a pollutant (e.g. benzene in ambient or microenvironment air) by the appropriate intake rate (e.g. breathing rate). The denominator requires some estimation of total emissions over a specified time period or emission rate. This may be derived from inventories or emissions models. Both measured or modelled values may be used in the numerator and denominator; however, one must be careful to note that measured values for the exposure concentration may include contributions from sources in addition to the source under investigation (e.g. the benzene concentration in urban air includes several source types, such as vehicles, industry, long range transport). Modelled values (e.g. from dispersion models) are more able to provide just the source contribution to exposure.
The numerator of iF requires an estimation of population or individual intake, derived from multiplying the media concentration or exposure of a pollutant (e.g. benzene in ambient or microenvironment air) by the appropriate intake rate (e.g. breathing rate). The denominator requires some estimation of total emissions over a specified time period or emission rate. This may be derived from inventories or emissions models. Both measured or modelled values may be used in the numerator and denominator; however, one must be careful to note that measured values for the exposure concentration may include contributions from sources other than the source under investigation (e.g. the benzene concentration in urban air includes several source types, such as vehicles, industry, long range transport). Modelled values (e.g. from dispersion models) are more able to specify only the source contribution to exposure.
Further details on applying the intake fraction methodology are given in the document on Exposure-intake models (see link below).
Rationale
An appealing aspect of iF is that it allows for an estimate of population exposure to a substance for which no exposure data are available, even new substances, as long as certain basic characteristics are known. Intake fraction tends to be relatively consistent and comparable across exposure pathways and source categories.
For example, studies have found the following general ranges for pathways:
- Inhalation dominant: range 1E-09 – 1E-05
- For primary PM2.5 typically 1E-06 – 1E-05
- Multipathway: range 1E-07 – 1E-05
- Ingestion dominant: range 1E-06 – 1E-04
Proximity of population to source and population density are also influencing factors, as the nearer the population to the source and the higher the density, the higher the intake fraction. For example, iFs of power plants are generally lower than those for vehicular emissions of primary particulate matter.
There are several assumptions and limitations to be aware of when using intake fraction. Intake fraction is subject to the same uncertainties associated with any modelling assessment (e.g. parameter uncertainty, model specification uncertainty). There is an assumption that the relationship between emissions and concentration (and intake) is linear. Intake fraction has been less frequently applied for reactive or secondary pollutants. Also, with an aggregate measure such as iF, one must be careful to include changes over time in the model. Equally, one needs to be careful not to double-count exposure as, for long-lived substances, the substance may recycle though the environment or, in the dioxins case, between mother and child. On the other hand, such recycling is still a part of the impact of the emissions that perhaps should not be ignored.
Examining the distribution of individual iFs or the spatial distribution might provide more information on the variability in a population or geographical area of the intake relative to the source emissions and could be useful in examining equity issues. Another issue that should be considered when estimating iFs is the background level as well as the dose-response of the substance.
It is important when using measured values to subtract out background values, which are not due to the source of interest. Also, the concept of iF is most useful with linear dose-response curves. For threshold or non-linear functions risks, iF may be poorly applicable for estimating population risks.
References
- Bennett, D.H., McKone, T.E., Evans, J.S., Nazaroff, W.W., Margni, M.D., Jolliet, O. and Smith, K.R. 2002a Defining intake fraction. Environmental Science and Technology 36, 207A-211A.
- Bennett, D.H., Margni, M.D., McKone, T.E. and Jolliet, O. 2002b Intake fractions for multimedia pollutants: a tool for life cycle analysis and comparative risk assessment. Risk Analysis 22, 905-918.
See also
Further information:
Examples of application:
- Intake fractions of cadmium emissions from a large zinc smelter
- Intake fraction: exposure to pesticides in food
